C02 at Mauna Loa, Hawaii
Global Average Temperature Courtesy of NASANow down to global warming. A few comments on this blog and a HUGE number of letters to the letters of local newspapers and online comments have asked the following question:
You say CO2 causes global warming. Well, CO2 has been going up the past ten years and the earth hasn't warmed during that period! Doesn't that mean that the global warming "theory" is wrong? What gives? Some of the more passionate comments go further, talking about liberal conspiracies, Al Gore, hoaxes, and places I don't want to go right now. But reasonable people ask this question...and there is a reasonable (although complex) answer.
Lets start with the data, shown above. Co2 and other greenhouse gases are going up steadily and have been as long as we have had direct measurements. We understand this. Mankind is the culprit. No responsible scientist doubts this. The other figure shows global temperature--generally going up, but there are are some periods of slight cooling or steady temperatures. Since the late 1990s, temperatures, albeit high, have shown little trend. This is what has some people concerned. But its worst that that they say. Some global warming computer simulations show warming during that period (see figure). Now we are really in trouble. Some of the models are failing too! Fox News was right! Mayor Nickels was wrong!
Not so fast. It turns out that global warming IS a real issue and the above doesn't prove anything really. The truth is that we have a signal to noise issue. The signal is the warming due to greenhouse gases. From our computer models and theory, we know that human-induced warming was quite small until recently (last few decades). And our models and theory indicate it will rapidly increase during this century. But there is also what I will call "noise"--natural variability in the atmosphere that obscures the global warming signal.
The atmosphere has all kinds of natural cyclic and non-cyclic phenomena that causes temperatures to vary even without any global warming. You know some of the them...El Nino and La Nina. Or the Pacific Decadal Oscillation. Or the Arctic Oscillation. There are too many to list. And the essential character of the atmosphere can produce variations on time scales of years to decades. Call this noise. And such noise obscures the global warming signal. When global warming is relatively weak (like now) the noise can even stop the warming or even cause temporary cooling. But it won't last. Eventually the noise will change sign (to warming) or the global warming signal will inevitably strengthen as greenhouse gases increases. Global warming will become apparent and more and more significant. There are also some other minor players to note...like solar variability (e.g, the sunspot cycle), which can produce slight reductions in solar radiation for a few years (like the past few).
During the past several years, global warming has undoubtedly lessened due to a combination of natural variability and a slight weakening of solar radiation, but you can bet that this will turn around very soon. (e.g., El Nino's bring warming and we are now switching into one). The fact that warming has paused for a few years really doesn't mean anything and doesn't disprove anything.
Finally, if you have my book there is a chapter on climate change, global warming, and its local implications.
